1. Field of the Invention
The present invention relates to an image sensor and more specifically to the pixel structure of such a sensor.
2. Discussion of the Related Art
FIGS. 1A and 1B are cross-section views of pixels of image sensors placed in integrated circuits comprising an interconnect network made of metal tracks and vias respectively made of aluminum and copper. The methods for manufacturing aluminum or copper interconnects are different. In the case of the pixel of FIG. 1A, corresponding to an “aluminum” technology, the tracks and the vias are separated from one another by the same insulating material, conventionally silicon oxide. In the case of the pixel of FIG. 1B, corresponding to a “copper” technology, an alternation of two types of insulating layers, conventionally silicon oxide and nitride layers, can be observed, the vias crossing oxide layers and the tracks being placed in openings of the nitride layers.
Each pixel is formed above a semiconductor substrate 1. A photosensitive area 2 is formed at the surface of substrate 1. The area of substrate 1 surrounding photosensitive area 2 is called peripheral area 3. Metal connections 4 are placed above peripheral area 3. Metal connections 4 are for example bit or row lines. In this example, the image sensors belong to an integrated circuit comprising six interconnect levels. Metal connections 4 are formed on the first two interconnect levels. The pixel shown in FIG. 1B, corresponding to a “copper” technology, comprises a stack Cb of insulating layers alternately formed of silicon oxide layers and of silicon nitride layers. The pixel shown in FIG. 1A, corresponding to an “aluminum” technology, comprises a stack Ca of six silicon oxide layers substantially forming in the end a single silicon oxide layer. For each pixel, a passivation layer 10 conventionally formed of a silicon oxide layer and of a silicon nitride layer covers stack Ca or Cb. Then, a filtering portion 11 covered with a planarizing layer 12 and with a lens 13 is placed above passivation layer 10.
An incident light beam on lens 13 converges towards photosensitive area 2 where it is “collected”. However, at each encountered surface of separation, part of the light beam is reflected and this, all the more as the refraction coefficients of the two materials in contact are remote.
In the case of the pixel shown in FIG. 1A, reflections can be observed at the surface of lens 13, at the contact surface between filtering portion 11 and passivation layer 10, at the intermediary surface between the oxide and nitride layers of passivation layer 10, and at the contact surface between stack Ca and photosensitive area 2.
In the case of the pixel shown in FIG. 1B, the same reflections as those previously described can be observed, as well as parasitic reflections at each of the interfaces between a silicon oxide layer and a silicon nitride layer of stack Cb.
Since the number of interconnect levels of an integrated circuit may be high, and tends to increase, the part of the observed parasitic reflections in the different interconnect levels of a pixel manufactured according to a “copper” technology becomes non-negligible. Further, due to the fact that the nitride layers of these interconnect levels are relatively thin, interference phenomena can be observed. The insulating layers of the different interconnect levels form an interference filter.
FIG. 2 is a diagram illustrating the light intensity received by photosensitive area 2 according to the wavelength of the incident light beams. In the case of a pixel in aluminum technology, the light intensity received by photosensitive area 2 very slightly varies, it is maximum for wavelengths located in the green field and slightly decreases towards red or blue. The maximum received intensity substantially corresponds to 80/90% of the intensity of all the incident light beams. In the case of a pixel in copper technology, significant variations in the intensity received by photosensitive area 2 can be observed according to the wavelength of the incident beams. In the visible field, an alternation of minimum intensities and of maximum intensities can be observed from blue to red. The maximum intensity values are slightly smaller than those sampled for a pixel in aluminum technology. The minimum intensity values are, as for them, very low and may reach a value substantially equal to ⅕ of the intensity of the incident light beams.
The development of integrated circuit manufacturing methods results not only in the above-mentioned problem, but also in the following problem. The increase in the number of functionalities on the same integrated circuit surface generally causes an increase in the number of interconnect levels placed above the semiconductor substrate. This results in an increase in the thickness of the assembly of layers placed between the photosensitive area and the converging lens placed at the top of the pixel. This causes problems of convergence of the received incident light beams.
FIGS. 3A and 3B are simplified cross-section views of two pixels belonging to integrated circuits exhibiting a different number of interconnect levels. Each pixel comprises a photosensitive area 30 formed at the surface of a semiconductor substrate 31. Substrate 31 is covered with an assembly of insulating layers 32, corresponding to the interconnect levels, and with a lens 33.
The convergence of lens 33 of a pixel is selected so that, for each pixel, incident light beams hitting lens 33 converge at the level of photosensitive area 30. The thickness of the assembly of layers 32 of the pixel shown in FIG. 3A being smaller than that of the pixel shown in FIG. 3B, the lens of FIG. 3B should be less converging that that of FIG. 3A.
Further, the incident light beams hitting a pixel may exhibit different inclinations with respect to the normal to the surface of substrate 31. The range of possible inclinations of the incident beams on a pixel depends on the objective placed above the image sensor and on the position of this pixel. The closer the pixel is located to the periphery of the pixel matrix, the more the average inclination of the light beams received by the pixel is significant.
In the case of FIG. 3A, inclined light beams, forming, for example, an angle from 20 to 30 with respect to the normal to the surface of substrate 31, converge at a point of the photosensitive area located substantially at the periphery thereof.
In the case of FIG. 3B, light beams having the same inclination converge towards a point of substrate 31 located outside of photosensitive area 30. Accordingly, the increase in the thickness of the assembly of layers 32 generates a loss of reception of inclined light beams, which decreases the pixel sensitivity. The sensitivity of a pixel even further decreases when, to integrate an ever-increasing number of pixels on the same surface, the surface of the photosensitive area of each pixel is decreased, as shown in dotted lines in FIG. 3B.
Another known problem of image sensors is the inter-pixel interference.
FIG. 4 is a simplified cross-section view of two adjacent pixels of an image sensor. As previously, each pixel comprises a photosensitive area 40, 41 formed at the surface of a substrate 42 above which is placed a stack of layers 43 corresponding to the various interconnect levels of the integrated circuit in which the image sensor is placed. Each pixel further comprises a filtering portion 44, 45 placed above stack 43. Filtering portions 44 and 45 transmit light beams having wavelengths in different ranges, such as green and red. Filtering portions 44 and 45 are conventionally covered with a planarizing layer 46 on which are placed lenses 47 and 48 at the top of each pixel.
In practice, the lenses of each pixel are not contiguous so that a portion of the surface of planarizing layer 46 is not covered. Since two adjacent pixels are not strictly delimited, incident inclined light beams may hit the surface of a pixel on an exposed area of planarizing layer 46 located close to the pixel lens, then cross the virtual border between the two pixels to reach the photosensitive area of the other pixel. Such light beams are in fact not “brought to account” by the right photosensitive area. Such accounting errors result in decreasing the image quality.